Motor keep on rotating for long time even if power off/motor pins short

Discussion in 'General Electronics Chat' started by Vindhyachal Takniki, Apr 8, 2016.

  1. Vindhyachal Takniki

    Thread Starter Member

    Nov 3, 2014
    348
    6
    1. I have purchaes a motor from local market. It don't have any part number on it(scratched out). Local market person syas it was used in earlier walkman.
    On internet I had checked my motor looks like this, but shaft is bit longer:
    http://www.aliexpress.com/item/hot-...tor-repetition-machine-motor/32263326923.html


    2. This motor runs even at 3.3V with very good speed, ther is no big load on it just very small tyre. Currently I am running at 12V, load is around 800mA by motor & its load.
    Problem is when motor is stopped, then keeps on rotating after that for long time. I want to stop motor at once when MCU stops PWM.
    I have alos tried by shorting the pins of motor manually, but even then it keeps on rotating.

    How to stop it at once?

    Edit: time change
     
  2. AnasMalas

    Member

    Nov 27, 2015
    66
    10
    It is still rotating because of inertia/momentum, because the motor likely is of high quality (all vintage sony products are) it has low internal friction, this means that there is nothing to stop it but a braking force.
    being a walkman motor, i assume i does not include electric breking because that isnt needed in a walkman, but does include reverse because you can reverse the tape. thus if you dont want to have an actual brake or actuator that only functions to stop he wheel, you could just apply current opposite to what you had been providing it forcing the motor to want to reverse, but the speed has to come to zero first! if the current is stopped just at the right time the wheel will just stop and not turn in the other direction.

    if you MCU supports flipping the polarity great, if not then just use an H-bridge IF THATS HOW YOUR MOTOR WORKS (which most do)
     
  3. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,526
    2,369
    If this were a simple DC P.M. motor, then shorting the leads should definitely brake the motor.
    Some of these small motors are now BLDC, so two leads indicate an internal circuit to provide the BLDC switching, so although it is P.M. , shorting the leads has no effect due to the internal circuit.
    Max.
     
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  4. AnasMalas

    Member

    Nov 27, 2015
    66
    10
    Shorting the leads means no current is passing the the motor. Thus no movement due to magnetism produced by the winding if it was a simple DC motor. if the system is of low friction with good quality bearings and brushes that provide low friction contact where would the breaking force be for such a small load?
    Even if it was an AC motor, or any type of motor, shorting has to be the easiest path taken shouldnt it? so no current is delivered to the motor thus no additional movement meaning that its using up its own inertia/Momentum.
    Am i wrong? please explain.
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
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    You will only get braking action with a motor that has a P.M. field, DC or BLDC etc, an AC induction motor for example will not brake with shorted leads.
    If you short the leads on a P.M. motor that is rotating or spun, the P.M. field enables the motor to act as a generator, and shorting the leads causes a high current due to this shorted generated voltage dependent on rpm.
    A P.M. motor is also generating when powered, the back EMF of which opposes the applied voltage.
    Max.
     
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  6. AnasMalas

    Member

    Nov 27, 2015
    66
    10
    Alright, makes sense. sorry if i sounded too invasive and somewhat rude....
     
  7. Tonyr1084

    Active Member

    Sep 24, 2015
    535
    86
    P.M. is a permanent magnet. Any motor with a PM will act as a generator if spun (or spinning). As generators go - if you load them they want to slow down. Dead shorting a generator (with a PM) will dead stop it.

    I have a DC motor in my garage that consists of a rotor coil and a stator coil. No permanent magnets. If I spin it manually it will not produce any current. Therefore, if it's not generating any current shorting the leads will prove ineffective.

    I don't know anything about the motor you have but I have messed with DC motors from computers, motors out of the hard drive (for instance). It DOES have a PM but it has electronic circuitry to control its speed and movement independent of the voltage input. Shorting such a motor will not stop the armature (rotor) because you're outside the motor leads. I'm not the expert on this subject, so if I'm wrong, someone will correct me. I don't mind that. In fact, learning from my mistakes has been a main way of learning. I've wasted a lot of smoke over the years.
     
  8. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,791
    1,103
    It's not wasted. IC manufacturers harvest it from the atmosphere and hermetically seal it inside their ICs, waiting to be let out when the ICs are powered up by unsuspecting end users :).
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,526
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    Another indication of the generation effect is to use one motor to drive another, e.g. if you couple two identical stepper motors in parallel and rotate one, the other will follow in synchro the same number of steps.
    The second has to have a very light load, as in an indicator etc.
    Max.
     
  10. Vindhyachal Takniki

    Thread Starter Member

    Nov 3, 2014
    348
    6
    Added a small mechanical braking system, now it is working fine.
     
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